A numerically controlled machine tool drills holes by controlling RPMs of a spindle and a feed axis so that the RPM of the spindle and an amount of feed commanded on a machining program as cutting conditions coincide with an actual RPM of the spindle and an actual amount of feed.
Incidentally, a tip configuration of a drill is gradually changed while drilling operation is repeated. Although proper cutting conditions are also sequentially changed accordingly, unless the conditions are positively changed on the machining program, drilling is carried out with a given amount of feed. Consequently, a load on the drill is gradually changed and holes may be defectively drilled or sometimes the drill may be broken.
Further, even if a load on a drill is very small, since an amount of feed is given, sometimes a drilling efficiency may be deteriorated.
To cope with this problem, there have been conventionally proposed adaptive control methods of preventing an unnecessary load from being applied on a drill.
For example, the applicant proposed in Japanese Patent Application Laid-Open No. 4-240011 an arrangement that a load sensor is disposed in an arbor mounted on a spindle to sense a load torque of a drill and when a detecting signal of the sensor is greater than a set load level, the drill is retracted once and drilling operation is continued again after changing an amount of feed.
In the conventional method, however, since an object to be controlled is a cutting feed speed (F) commanded by a program and the load torque of the drill is indirectly controlled by controlling the cutting feed speed F, it cannot be always said that a load on the drill is properly controlled.
Further, a load torque imposed on a drill is greatly varied by factors such as a depth of a drilled hole, and an amount of chips produced between a drill groove and the inner wall of the drilled hole. In the above conventional method, since an upper limit is set to the load torque and the cutting feed speed F affecting the load torque is positively changed, sometimes the load torque is too abruptly varied and the drill may be broken before the abnormal torque is sensed and the drill is retracted.
On the contrary, when a load is set to a sufficiently low level, the cutting feed speed F must be made small accordingly. As a result, a longer time is needed to drill holes and a drilling efficiency is deteriorated.
Further, since a command of the cutting feed speed F is automatically changed, it is difficult for the operator to get a correct value of the cutting feed speed F at any given time.